Carrier mobility is one of the fundamental physical quantities which largely impact the device performance. In this regard, the knowledge of the carrier mobility is indispensable for the refinements of material processing as well as device design. However, for amorphous semiconductors including amorphous silicon alloys (a-Si) which are of great interest in solar cell field, no simple means have been available for the direct measurement of the "free" carrier mobility, instead it is conventionally deduced from the temperature dependence of the TOF "total charge-carrier" mobility by relying on some specific models. The present work aims to establish a new tool for the determination of free carrier mobility near the band edge by the use of polarized electroaborption and Hall measurements, and to investigate how mobilities are affected by alloying, doping, preparation conditions and/or various treatments after preparation. Upon Phosphorous doping, the electron mobility decreases to about one-third of that in undoped case, while the hole mobility remains almost unchanged. If the long-range potential fluctuation plays a critical role in determining the transport property, then both the electron and hole mobilities should be equally reduced by the incorporation of charged impurities and/or defects, which is, however, in contradiction with our experimental observation. Furthermore, we have found on B doped materials a qualitatively identical behavior of mobilities ; drop in the electron mobility, but no significant reduction in the hole mobility. These findings seem to imply that the doping-induced change in mobilities should not be attributed to classical charge effects, but the incorporation of impurity atoms itself may be of central importance.